This invention relates generally to a code conversion system which effects compressed encoding of a sampled multi-level time series signal subject to a variation of occurrence frequency of levels and decoding of the same.
The multi-level time series signal which is obtained by sampling a video or voice signal generally involves a variation of occurrence frequency of the individual multi-levels.
The pulse code modulation (PCM) signal which is obtained by A/D conversion of an analog video signal or voice signal has no appreciably large variation of occurrence frequency, whereas the multi-level signal which is obtained by converting a PCM signal into a differential pulse code modulation (DPCM) signal generally exhibits an occurrence frequency distribution of the zero-concentrated form, namely a form in which signals having differential levels approximating 0 (zero) occur at a high occurrence frequency and signals having large differential levels occur at a low occurence frequency. The signal having such a variation of occurrence frequency as described above can be subjected to compressed coding by the variable length coding which allocates short length codes to signals involving high occurrence frequency and long length codes to signals involving low occurrence frequency. This variable length coding is a method which gives to the sampled multi-level differential time series signal such variable length codes as are predetermined in accordance with differential signals existing at individual sampling times. In this case, at least one bit of code is required for the differential level of the highest occurrence frequency. In an interframe DPCM coding to be effected on video signals such as for television, however, it is usual that the probability of the occurrence of signals having a zero differential level reaches 90% or more. Thus, allocating the one-bit code to the signals of the zero differential level proves to be inefficient. An attempt has been made, therefore, to improve the compression efficiency as by dividing the time series signal subject to a heavy variation of occurrence frequency into unit blocks each containing eight sampled levels, for example, and assigning a special code to a unit block in which all the eight signal levels have the highest occurrence frequency (hereinafter referred to as "highest occurrence frequency signal levels"). This approach, however, has a disadvantage that the compression efficiency is not so improved for the complexity of system configuration inevitably entailed.